1. Field of the Invention
The present invention is concerned with a combination isomerization/alkylation process. More particularly, the invention relates to isomerizing a mixed butenes-containing feed such that at least a portion of the butene-1 is converted to butene-2 prior to alkylating an isoparaffinic hydrocarbon with at least a portion of the isomerized feed to produce more highly branched hydrocarbons.
2. Description of the Prior Art
The use of catalytic alkylation processes to produce branched chain hydrocarbons having valuable anti-knock properties that are suitable for use as gasoline blending components is well known in the petroluem refining art. Generally, the alkylation of saturated hydrocarbons, such as isoparaffins, with olefins is accomplished by contacting the reactants with an acid catalyst, such as sulfuric acid, fluorosulfuric acid or a halogen acid, such as hydrofluoric acid, to form a reaction mixture, settling said mixture to separate the catalyst from the hydrocarbons, and further separating the hydrocarbons, for example by fractionation, to recover the alkylation reaction product. The alkylation reaction product is normally a mixture of C.sub.5 -C.sub.10 paraffins, often termed "alkylate", and typically contains a mixture of C.sub.8 -C.sub.9 hydrocarbons, the composition of which depends upon the particular isoparaffinic and olefinic reactants utilized. The formation of more highly branched hydrocarbons, e.g., trimethylpentanes, rather than less branched hydrocarbons, e.g., dimethylhexanes, is preferred because the former provide a higher octane gasoline blending stock.
The formation of an alkylate of improved quality by isomerizing a mixed butenes-containing feed (one containing butene-1, butene-2, and isobutene), i.e. shifting the double bond in the organic molecule from a terminal position to a more central position such as occurs when forming butene-2 from butene-1, prior to alkylating a paraffin, particularly an isoparaffin, with the resulting isomerate has also been suggested for several alkylation catalysts, particularly sulfuric acid and hydrofluoric acid (see, for example, U.S. Pat. Nos. 2,377,352; 2,429,205; 2,450,039; 2,460,303; 2,502,015; 2,591,367; 2,594,393; 3,763,261, and 3,800,003). As an example, Example II of U.S. Pat. No. 2,591,367 contains the following data for alkylation in the presence of 96% sulfuric acid:
______________________________________ Untreated Feed Isomerized Feed ______________________________________ Composition of Feed Propane-propylene 0.4 -- Isobutene 5.1 0.1 Butene-1 4.4 1.5 Butene-2 4.9 10.2 Isobutane 71.5 53.4 Normal butane 13.7 34.8 Quality of Product Octane No. of Alkylate 92.5 93.7 ______________________________________
However, as will be discussed hereinbelow, the substantial removal of isobutene, which produces a relatively low octane alkylate, i.e. 85-90 unleaded motor octane number (MONC), from the isomerized feed could well have produced a large portion of the increased octane number rather than the isomerization of butene-1 to butene-2.
In the case of hydrofluoric acid, Examples I and II in U.S. Pat. No. 2,502,015 indicate that the alkylate produced from reacting isobutane and butene-1 without a preliminary isomerization step has an ASTM octane number of about 88.9. This compares with an octane number of about 92.5 obtained when the butene-1 present in the effluent from the isomerization step is removed therefrom prior to alkylation. In addition, the data in Examples IV and V indicate that alkylation with butene-2 rather than butene-1 provides an octane number advantage of 3.5 and 3.8, respectively. Thus, the teachings of the U.S. Pat. No. 2,502,015 indicate that the difference in alkylate quality between butene-1 and butene-2 ranges from about 3.5 to about 3.8 octane. Similarly, if the octane number and feed composition data in U.S. Pat. No. 2,594,343 is extrapolated to the pure isomers, the octane numbers obtained for butene-1 and butene-2 are 92.6 and 95.8, respectively, a difference of 3.2.
A similar result may be obtained by comparing Runs 2 and 5 of Table 1 in U.S. Pat. No. 3,800,003. As separate feedstocks, butene-1 and isobutene provided alkylates which when blended together in equal amounts gave 94.1 MONC. As separate feedstocks, butene-2 and isobutene gave alkylates which when blended together in equal amounts gave 96.2 MONC. Assuming the contribution from isobutene was the same in each case, the alkylate formed from butene-2 was about 4.2 MONC better than that from butene-1.
The above data indicate that the alkylate produced using butene-2 is from 3-4 MONC superior to that obtained with butene-1 for hydrofluoric acid alkylation. Thus, it would be expected that converting butene-1 to butene-2 prior to hydrofluoric acid alkylation would produce an improved alkylate.
However, it has been found that for the alkylation process described in U.S. Pat. No. 3,887,635, the disclosures of which are incorporated herein by reference, isomerization of a mixed butenes-containing feed prior to alkylation provides an improved alkylate even though alkylation of butene-1 or butene-2 in said process produces an alkylate of substantially equivalent octane number.